1. Field of the Invention
This disclosure relates to an inkjet recording apparatus provided with a liquid surface level sensing unit configured to sense a liquid surface level change in a reserve tank arranged in a flow channel extending from an ink tank to a recording head.
2. Description of the Related Art
In recent years, the amount of ink consumed per sheet has increased due to demands such as high image quality and wide-format printing, such that the ink in an ink tank of an inkjet recording apparatus often runs out during a recording operation.
In order to solve the problem of running out of ink, an inkjet recording apparatus provided with a reserve tank in an ink flow channel between the ink tank and a recording head is proposed.
For example, in the inkjet recording apparatus illustrated in FIG. 13A, an ink tank 5 is connected to a recording head 1 via a first hollow tube 8, a reserve tank 4, and a supply tube 2. Even when the ink tank 5 is out of ink, the recording operation can be temporarily continued with ink in the reserve tank 4, and the ink tank 5 can be replaced by a new tank while continuing the recording operation (continuous recording).
In an inkjet recording apparatus having such a configuration, the amount of ink in the reserve tank needs to be constantly monitored during the recording operation. When a reduction in the amount of ink in the reserve tank is sensed, it is estimated that the ink tank is empty, and a user is prompted to replace the ink.
The amount of ink in the reserve tank (the amount of ink remaining in the reserve tank) is detected by sensing that the ink surface is below a predetermined position (H) (sensing of the ink surface level). Exemplary embodiments of the liquid surface level sensing unit used for sensing the ink surface level include electrodes (421 and 422) configured to sense a potential difference and an optical sensor using light reflection properties as illustrated in FIG. 13A.
However, air bubbles generated in the ink tank, or air bubbles entering a liquid flow channel during replacing the ink tank may flow into the reserve tank together with a flow of ink. If the air bubbles flow into the reserve tank, the liquid surface level sensing unit may not work correctly, that is, erroneous sensing may occur.
For example, in the case of liquid surface level sensing by using electrodes, an energized state between two electrodes is not released due to air bubbles which have accumulated on the liquid surface, such that a liquid surface level change (lowering) cannot be sensed even though the liquid surface level is lowered.
The same applies to the case of liquid surface level sensing by using an optical sensor. In other words, a light beam is reflected due to the presence of the air bubbles adhering to the surface of a prism of the optical sensor irrespective of the presence or absence of the liquid (ink), such that the liquid surface level change may not be sensed.
As a countermeasure for the erroneous sensing problem due to the air bubbles as described above, a disclosure in Japanese Patent Laid-Open No. 2007-237552 is proposed.
Specifically, according to Japanese Patent Laid-Open No. 2007-237552, an internal space of the reserve tank open to the atmosphere is partitioned into two chambers by a partitioning plate. An ink inlet port is provided on a lower portion of one of the chambers. The partitioning plate is provided with communication ports to make the two chambers communicate with each other at two positions, namely, above and below the ink surface level in order to keep the liquid surface levels in the chambers on both sides of the partitioning plate the same. With the provision of an optical sensor in a chamber different from the chamber provided with the ink inlet port, a method is proposed to prevent erroneous sensing by blocking air bubbles entering through the ink inflow port to keep the air bubbles and the optical sensors out of contact with each other.
If an attempt is made to provide a partitioning plate 41 disclosed in Japanese Patent Laid-Open No. 2007-237552 in the reserve tank 4 illustrated in FIG. 13A, the partitioning plate 41 needs to be provided with communication ports 41A and 41B as illustrated in FIG. 13B.
In a configuration illustrated in FIG. 13B, an ink inflow port 8a through which the ink flows from the ink tank 5 to the reserve tank 4 is provided on an upper surface (top surface) of the reserve tank 4. Therefore, when ink flows into the reserve tank 4 up to the upper surface (the reserve tank 4 is filled with ink up to the top), no space remains in which air can exist.
If ink in the ink tank 5 is used up in a state in which the reserve tank 4 is filled with ink to the top surface, air (air bubbles) flows into the reserve tank 4 from the ink tank 5. In other words, air (air bubbles) having the same volume as a volume of the ink which has flowed out from the reserve tank 4 toward a head flows into the reserve tank 4 from the ink tank 5.
In this case, the ink surface level in the reserve tank 4 is lowered. However, a volume (height) of air bubbles which have flowed from the ink tank 5 and accumulated on the liquid surface is increased by an amount not smaller than the amount of lowering of the ink surface level in the chamber provided with the ink inflow port 8a. Consequently, as illustrated in FIG. 13B, there is an increased risk (probability) that air bubbles enter the chamber provided with the electrodes (421 and 422), which are liquid surface level sensing units, through the upper communication port 41B and adhere to the electrodes.
Therefore, even though the liquid surface level (the amount of ink remaining in the reserve tank 4) in the reserve tank 4 is lowered, an electricity conducting state between the electrodes (421 and 421) due to the air bubbles which have entered the chamber provided with the electrodes (421 and 422) and accumulated on the liquid surface cannot be resolved. Therefore, there is a probability that lowering of the liquid surface level in the reserve tank cannot be sensed accurately (may be erroneously sensed).